Field of the Invention
The teachings herein are directed to systems and methods for distracting an intervertebral disc space using a staged, bilaterally expandable trial.
Description of the Related Art
Bone grafts are used in spinal fusion, for example, which is a technique used to stabilize the spinal bones, or vertebrae, and a goal is to create a solid bridge of bone between two or more vertebrae. The fusion process includes “arthrodesis”, which can be thought of as the mending or welding together of two bones in a spinal joint space, much like a broken arm or leg healing in a cast. Spinal fusion may be recommended for a variety of conditions that might include, for example, a spondylolisthesis, a degenerative disc disease, a recurrent disc herniation, or perhaps to correct a prior surgery.
Bone graft material is introduced for fusion and a fusion cage can be inserted to help support the disc space during the fusion process. In fact, fusion cages are frequently used in such procedures to support and stabilize the disc space until bone graft unites the bone of the opposing vertebral endplates in the disc space. A transforaminal lumbar interbody fusion (TLIF), for example, involves placement of posterior instrumentation (screws and rods) into the spine, and the fusion cage loaded with bone graft can be inserted into the disc space. Bone graft material can be pre-packed in the disc space or packed after the cage is inserted. TLIF can be used to facilitate stability in the front and back parts of the lumbar spine promoting interbody fusion in the anterior portion of the spine. Fusion in this region can be beneficial, because the anterior interbody space includes an increased area for bone to heal, as well as to handle increased forces that are distributed through this area.
Unfortunately, therein lies a problem solved by the teachings provided herein. Currently available systems can be problematic in that the methods of introducing the fusion cage and bone graft material creates “subsidence” of the cage into the adjoining vertebrae, resulting in a narrowing of the formerly distracted disc space. This is because the cage is inserted near the middle of the endplate area which is softer than the areas at or near the peripheral zone of the endplate, and when it distracts, the cage actually sinks into the endplate creating the subsidence problem. The problem remains with state-of-the-art distraction instruments, such as the Medtronic SCISSOR JACK, paddle trials, or oversized trial shims (metallic wedges). Each of these state-of-the-art procedures introduce the distraction means narrowly (no wider than width of annulotomy) and then distract the intervertebral space with a narrow foot print that ranges from about 8 mm to about 11 mm wide.
Accordingly, and for at least the above reasons, those of skill in the art will appreciate distraction systems that facilitate an improved placement of distraction stresses across the verterbral endplates that define the distracted intervertebral space. Such systems are provided herein, the systems configured to (i) effectively and selectively place the distraction stresses in areas that include areas at or near the peripheral zones of the vertebral endplates to reduce the incidence of subsidence; (ii) reduce or eliminate the problem of failures resulting from subsidence; (iii) have a small maximum dimension of the trial in a collapsed state for a low-profile insertion into the annulus in a minimally-invasive manner, whether using only a unilateral approach or a bilateral approach; (iv) laterally expand within the intervertebral space to facilitate the effective and selective distribution of distraction stresses on the vertebral endplates; (v) vertically expand for distraction of the intervertebral space; (vi) provide an expansion in the intervertebral space without contracting the system in length to maintain a large footprint during the distraction process, distributing load over a larger area, including areas at or near the peripheral zones of the vertebral endplates; and, (vii) serve as a measuring device for the size of the intervebral space to facilitate selection of the size of the cage.